The invention relates to a conjugated polymer in an oxidized state and a method of preparing such a conjugated polymer.
The invention further relates to a substrate surface, in particular a surface of a display screen of a display device, provided with a layer comprising such a conjugated polymer.
The invention also relates to an electrode comprising such a conjugated polymer, and in particular to an electroluminescent device comprising such an electrode.
In the context of the invention, a conjugated polymer is a polymer (the term polymer includes copolymer, terpolymer, etc. as well as oligomer) which has a poly-conjugated system, i.e. an extensive interconnected system of double and/or triple bonds (also known as .pi.-bonds or unsaturated bonds). Generally, the poly-conjugated system extends along the main chain (backbone) of the polymer.
As is well known in the art, the presence of a poly-conjugated system renders many conjugated polymers readily oxidizable. Thus, a conjugated polymer in a neutral state may be brought into an oxidized state by bringing said conjugated polymer into contact with an oxidative agent, whereupon a redox process occurs. As a result, a conjugated polymer in an oxidized state having an increased specific conductivity is obtained which is interesting from a technical point of view. Typical values range from 10.sup.-3 to 10.sup.3 S/cm. The redox process is also referred to as doping, the oxidative agent as a dopant, and the oxidized state as a doped state, whereas the conjugated polymer in an oxidized state is also referred to as a doped conjugated polymer, for short. If the charge carriers accommodated by the poly-conjugated system are positively charged, the doping is said to be p-type. If the poly-conjugated system carries almost no charge, the conjugated polymer is in the neutral or undoped state.
Layers comprising such doped conjugated polymers are electroconductive and can be suitably used for a large variety of applications. For example, a layer of such a polymer can be used as an anti-static or electromagnetic shielding coating or as an electrode layer.
Doping the conjugated polymer does not only change the electrical properties but also the optical properties associated with the conjugated polymer, such as its color and transparency in the visible range. As a general rule, it appears that conjugated polymers are, both in the doped and undoped state, highly colored and rather opaque.
This is unfortunate since in the art a clear need exists for polymers which are electroconductive, transparent to visible light and, preferably, colorless. Such polymers can be suitable used in place of or in addition to known inorganic transparent conductors such as indiumtinoxides. However, attempts to satisfy this need have been largely unsuccessful. More in particular, as exemplified by a publication by Having a et al. in Chem. Mater., vol. 8, 1996, pp 769-776, it has so far been impossible to provide a conjugated polymer in an oxidized state which is electroconductive, transparent and, preferably, colorless, if said polymer, in its neutral, state has an oxidation potential, expressed in terms of a half-wave potential, E.sub.1/2, at room temperature, of more than 0.0 V vs the saturated calomel electrode (SCE). In this respect it is noted that both poly-3,4,ethylenedioxythiophene (PEDOT) and polyaniline (PANI), which are among the very few polymers which are known to be transparent and electroconductive in the oxidized state, have an oxidation potential in the neutral state which is significantly lower than 0.0 V vs SCE, where it is understood that the neutral undoped state of PANI is leucoemeraldine PANI. PANI is an exceptional conjugated polymer in that it has another neutral form, which is known as emeraldine base PANI.